1. Field of the Invention
The present invention relates to an apparatus for and a method of detecting deterioration of a catalyst in an internal combustion engine and, more particularly, to improving the accuracy of catalyst deterioration diagnosis for an internal combustion engine.
2. Description of the Related Art
A technique in which a means for changing the air-fuel ratio to detect deterioration of a catalyst sets the changing range so that the amount of oxygen storage is within the range between a breakthrough amount of an aged catalyst (i.e., oxygen storage capacity of the catalyst) and a breakthrough amount of a normal catalyst, has been proposed (see, for example, Japanese Patent Laid-Open No. 2002-130018). The amount of oxygen storage is calculated by detecting the concentration of oxygen in exhaust gas with an oxygen sensor provided downstream of the catalyst.
In such a conventional catalyst deterioration detecting apparatus for internal combustion engines, however, the amount of oxygen storage is indeterminate when detecting deterioration of a catalyst is started and there is a possibility of a substantial output variation of the oxygen sensor even when the catalyst is normal and, hence, failure to accurately detect deterioration of the catalyst.